1. Field of the Invention
The present invention relates generally to synchronizing communication among plural nodes in a communications system and more particularly, to synchronizing communication among plural nodes which communicate within a network having a wireless backbone of repeaters.
2. State of the Art
Communication systems, such as local area networks (LANs), have achieved widespread use for the exchange of information. These systems include a plurality of data processing nodes or "agents", which access a transmission medium that interconnects the nodes. Many local area network schemes have been devised for providing the nodes with a shared access network, or backbone, through which the nodes communicate with each other or with a central hub, or server. Where plural nodes of a network are synchronized to a central hub, local area network schemes have been devised with a central hub organization wherein a backbone of nodes which function as repeaters are interconnected by a wired communication path. Non-repeater nodes are connected to repeater nodes which are directly, or via other repeater nodes, connected to the central hub. These network arrangements typically involve hardwire connections.
More recent efforts have been directed to developing wireless communication networks. For example, commonly assigned U.S. Pat. No. 5,231,634 (Giles et al) relates to a communication system for regulating communication among a plurality of nodes, each of which is equipped with a radio transceiver. The '634 patent describes medium access control of a wireless LAN to provide decentralized, peer-to-peer access for all of the agents. A peer-to-peer communication system is one in which all agents can freely communicate without any agent having an advantage over another.
Wireless communication systems require consideration of potential problems which do not exist in hardwired systems. For example, in a wireless communication system, the nodes and repeaters can be mobile, moving in and out of range of other nodes and repeaters in the system. The '634 patent is directed to continuously ensuring that all agents possess fair access to the communication system (i.e., to avoid, for example, agents which are physically closer to a given destination from having an unfair advantage due to their signal strength being greater than that of agents physically located further from the destination).
Wireless systems also require consideration of radio transmission limits imposed by a physical setting, or even by FCC regulations regarding radio emissions. Due to these considerations, wireless networks, such as the one described in the '634 patent, may have restricted operating flexibility.
Due to their unique considerations, wireless communication systems have not been developed which can freely exploit communication techniques such as central hub organization. Central hub organization is a network scheme involving use of a central, or master repeater, that controls the operation of the network. The master repeater may communicate directly with non-repeater nodes or, in widely distributed networks, through the use of repeater nodes. For purposes of this discussion, both repeater and non-repeater nodes are referred to as "nodes" while repeaters are also referred to as "repeaters". The repeaters in a master repeater network facilitate the propagation of data, control and synchronization information to the nodes, including other repeaters. The repeaters may be in direct contact with the master repeater or, may be in contact with another repeater that is directly or indirectly in contact with a master repeater.
Because of the constraints associated with wireless network schemes, such schemes have at most been used for communications between the nodes and the repeaters of a central hub architecture and not for communications between repeaters. By limiting wireless communications to communications between nodes and repeaters, interference of signals within the overall network can be controlled. For example, all nodes which communicate to the central hub via a first repeater can communicate with the first repeater on a first frequency channel, while all nodes which communicate with the central hub via a second repeater can communicate with the second repeater on a second frequency channel. The nodes connected to a given repeater thus constitute a sub-network, or domain. Each of the repeaters can then communicate with the central hub via a wired link through any number of repeaters to ensure reliable communications. Because repeaters typically handle increased communications (i.e., communications from nodes connected directly to the repeater as well as communication to and from linked repeaters) reliable communication between repeaters can thus be ensured with wired links.
Although techniques exist for enhancing the reliability of wireless communications, these techniques have not been considered practical for use in interconnecting a backbone of repeaters. For example, frequency hopping is a known spread-spectrum technique whereby a signal is transmitted using a plurality of frequencies. The exact frequency used at any given time is switched from one frequency to another either in a random or a predetermined sequence. Frequency hopping is useful in preventing fading errors, and is generally more secure than single channel communication.
While frequency hopping techniques provide reliable and secure wireless communication, conventional frequency hopping techniques are not readily adaptable to a wireless LAN environment, and in particular, a wireless LAN environment which includes a backbone of repeaters which can move into and out of any number of dynamically changing, reconfigurable arrangements. Further, the need to maintain a high level of synchronization for wireless communication has been considered to render its use impractical for high traffic repeaters which are located in communication networks that operate with a number of different frequency channels (e.g., a different frequency channel for each repeater).
It would therefore be desirable to provide the advantages of a central hub organization which exploits the use of wireless links for communication between nodes and repeaters, and for communication between repeaters and a central hub. Further it would be desirable to provide some mechanism for establishing and maintaining synchronization among the repeaters in a dynamically changing, reconfigurable communication system.